Compress the overall dynamic range of the audio using a compressor. Each compressor has a different sound, but all have the same basic functions.
Set the "threshold" of the compressor so that it turns on during the louder passages of the audio. When the audio level passes the level set by the compressor, the compressor will turn on. The more you compress the audio, the louder you'll be able to get it. However, compressing the audio too much can cause it to sound squashed.
Set the "ratio" of the compressor to determine how much you want to compress the audio. Typically, a low compression ratio such as 2-to-1 is used for mastering. The lower the ratio, the more natural and transparent the compression sounds. For example, a compression ratio of 2-to-1 will allow the volume to increase one decibel for every two decibels the audio level goes above the threshold.
Set the "attack" control of the compressor to set how quickly the compressor turns on once the threshold is crossed. A fast-attack setting will allow the compressor to turn on very quickly, while a slow-attack setting will result in the compressor reacting much slower when the threshold is crossed. Experiment with the setting of this control to hear how it affects the sound of your audio.
Set the "release" control of the compressor to set how quickly the compressor turns off once the volume falls below the threshold. As with the attack control, a fast-release setting will allow the compressor to turn off very quickly, while a slow-release setting will result in the compressor reacting much slower when the audio level falls below the threshold. Experiment with the setting of this control to hear how it affects the sound of your audio.
Adjust the "Makeup Gain" of the compressor to make up for volume lost during compression. For example, if the compressor is reducing the gain by four decibels at the loudest section of the audio, you should turn the makeup gain control up four decibels. This will raise the overall volume of the audio without actually increasing the peak volume level of the audio.
Limit the overall dynamic range of the audio using a limiter after you have compressed it. A limiter works similar to a compressor but is usually the last unit in a processing chain. It is used to limit, or stop, the peaks of the audio from going above a certain level. As with compressors, each limiter has a different sound. However, most limiters have the same basic controls.
Set the threshold of the limiter so that the limiter turns on at the very loudest peaks of the audio. The lower you set the threshold, the more peaks it will limit and the louder you'll be able to make the overall level of the audio. However, too much limiting can cause the audio to distort and sound bad.
Increase the "Makeup Gain" control to make up for the gain lost through limiting. Some limiters will automatically make up for this gain. If yours does not, increase the makeup gain the same number of decibels by which the limiter decreases your audio level when limiting its peaks.